A Miami-bound Amtrak train collided with a CSX freight train around 2:35 a.m. Sunday, killing two and injuring at least 116. The accident occurred less than a week after an Amtrak train carrying Republican lawmakers collided with a garbage truck, killing the driver, in West Virginia, and seven weeks after the accident north of Olympia, Washington that killed three people. And while no one knows exactly why the most recent accident happened, some experts have already said this most recent crash—and the one in Washington—could have been prevented if the trains were using a technology known as positive train control, or PTC. In fact, more than 140 accidents, nearly 300 fatalities, and more than 6,500 injuries could have been prevented by PTC, according to the National Transportation Safety Board, the government agency responsible for investigating civil transportation accidents.

“How many years have we been calling for PTC? PTC is designed to mitigate mistakes like this," said Robert Sumwalt, the chairman of the NTSB, after the most recent crash, according to the New York Times. "This is, indeed, a human mistake.”

What is PTC?

It's a system designed to stop a train before an accident occurs. Trains use an onboard computer system to monitor track conditions and speed limits along their route, sending engineers warnings when conditions are out of the ordinary. If the train engineer doesn't react to those warnings within a specified window of time, PTC automatically applies the brakes. “It's very effective in train collision prevention and over-speed prevention," Dr. Allan Zarembski, director of the Railroad Engineering and Safety Program at the University of Delaware, has previously said. "[But] it is not a cure-all—it does not address accidents or derailments caused by track or equipment failures." PTC also can't prevent vehicle-train collisions at crossings, like last week's collision in West Virginia.

What percent of trains and tracks have PTC?

Currently, 49 percent of Amtrak's locomotives and 67 percent of its tracks are equipped with PTC. Congress passed a law in 2008 requiring PTC on all main railroad lines by 2015, but, in late 2015, that deadline was extended to December 31, 2018. (Railroads can get an extension lasting until December 2020, at the latest.) Transportation Secretary Elaine L. Chao has said the railroad industry will be held to the deadline. "We are concerned that many of the nation’s railroads must greatly accelerate their efforts to achieve the congressionally mandated requirements," she wrote to one rail executive in a December 2017 letter.

How much will this all cost?

One of the biggest hurdles to full PTC implementation has been financial. It could cost up to $22.5 billion over the next 20 years; Congress has already invested $6.5 billion. Still, railroad experts say it must be implemented, no matter the cost: "Without it," the NTSB wrote in 2015, "everybody on a train is one human error away from an accident."